Helicopter manufacturers normally use a combination
of a stabilization and/or AFCS in order to meet the IFR
stability requirements of Parts 27 and 29. These systems
include:

Aerodynamic surfaces, which impart some stability
or control capability that generally is not
found in the basic VFR configuration.

Trim systems, which provide a cyclic centering
effect. These systems typically involve a magnetic
brake/spring device, and may be controlled
by a four-way switch on the cyclic. This system
supports “hands on” flying of the helicopter.

Attitude Retention Systems (ATTs), which
return the helicopter to a selected attitude after a
disturbance. Changes in attitude can be accomplished
usually through a four-way “beep”
switch, or by actuating a “force trim” switch on
the cyclic, which sets the desired attitude manually.
Attitude retention may be a SAS function, or
may be the basic “hands off” autopilot function.

Autopilot Systems (APs) provide for “hands off”
flight along specified lateral and vertical paths.
The functional modes may include heading, altitude,
vertical speed, navigation tracking, and
approach. APs typically have a control panel for
mode selection and indication of mode status.
APs may or may not be installed with an associated
flight director (FD). APs typically control
the helicopter about the roll and pitch axes (cyclic
control) but may also include yaw axis (pedal
control) and collective control servos.

Flight Directors (FDs), which provide visual
guidance to the pilot to fly selected lateral and vertical
modes of operation. The visual guidance is
typically provided by a “single cue,” commonly
known as a “vee bar,” which provides the indicated
attitude to fly and is superimposed on the attitude
indicator. Other flight directors may use a “two
cue” presentation known as a “cross pointer system.”
These two presentations only provide attitude
information. A third system, known as a
“three cue” system, provides information to position
the collective as well as attitude (roll and
pitch) cues. The collective control cue system
identifies and cues the pilot which collective control
inputs to use when path errors are produced, or
when airspeed errors exceed preset values. The
three-cue system pitch command provides the
required cues to control airspeed when flying an
approach with vertical guidance at speeds slower
than the best-rate-of-climb (BROC) speed. The
pilot manipulates the helicopter’s controls to satisfy
these commands, yielding the desired flight
path, or may couple the autopilot to the flight
director to fly along the desired flight path.
Typically, flight director mode control and indication
are shared with the autopilot.

Pilots must be aware of the mode of operation of the
augmentation systems, and the control logic and functions
in use. For example, on an ILS approach and
using the three-cue mode (lateral, vertical and collective
cues), the flight director collective cue responds to
glideslope deviation, while the horizontal bar cue of
the "cross-pointer" responds to airspeed deviations.
However, the same system when operated in the twocue
mode on an ILS, the flight director horizontal bar
cue responds to glideslope deviations. The need to be
aware of the flight director mode of operation is particularly
significant when operating using two pilots.
Pilots should have an established set of procedures and
responsibilities for the control of flight director/autopilot
modes for the various phases of flight. Not only
does a full understanding of the system modes provide
for a higher degree of accuracy in control of the helicopter,
it is the basis for crew identification of a faulty
system.

A “sensitive” altimeter relates to the instrument's displayed change in altitude over its range. For “Copter” Category II operations the scale
must be in 20-foot intervals.